Singer 348 Service Manual

The Singer 348 is a classic straight-stitch sewing machine manufactured in the mid-20th century, known for its durability and mechanical simplicity. This service manual provides comprehensive instructions for maintenance, adjustment, troubleshooting, and repair of the Singer 348 model. Below are key sections covering identification, lubrication, timing, stitch regulation, tension, and common repairs.

Machine Identification & Specifications

Key identifiers: Model number 348, Class 66 bobbin, oscillating hook, built-in motor, reverse stitch lever.

Feature	Description
Model Number	348 (typically found on rear bed plate)
Stitch Type	Straight Stitch Only
Bobbin Type	Class 66 Metal Bobbin
Hook System	Oscillating Hook
Stitch Length	Adjustable via dial (0-4 mm)
Reverse Stitch	Lever-activated
Motor	Built-in, belt-driven
Needle System	15x1 (HAx1) Household Needle
Weight	Approximately 25 lbs (11.3 kg)
Production Era	Circa 1950s-1960s

Safety Precautions

Always unplug the machine before performing any service or maintenance.

1. Disconnect Power: Unplug from electrical outlet before starting any work.
2. Secure Machine: Place machine on a stable, non-slip surface.
3. Use Proper Tools: Employ correct screwdrivers and wrenches to avoid damage.
4. Handle Needles with Care: Always point needle away from yourself and others.

WARNING! The machine is heavy; lift with proper technique. The needle is sharp and can cause injury.

Disassembly & Reassembly

General guide for accessing internal components.

1. Remove Outer Casing: Unscrew and lift off the top cover and side plates.
2. Remove Face Plate: Unscrew small screws to access tension assembly and take-up lever.
3. Access Hook Area: Remove slide plate and bobbin case.
4. Remove Motor Belt: Loosen motor mounting screw to slacken belt for removal.
5. Reassembly is the reverse order. Ensure all parts move freely before final tightening.

CAUTION! Keep screws organized. Do not force parts. Refer to diagrams for correct orientation.

Lubrication Guide

• Oil Type: Use only high-quality sewing machine oil (clear, non-gumming).
• Frequency: Oil after every 8-10 hours of use or as part of annual service.
• Key Points:
  1. Apply one small drop to each bearing point on the main shaft.
  2. Oil the hook race and oscillating hook post.
  3. Lubricate the take-up lever pivot and presser bar lifter joints.
  4. Run the machine without thread for a minute to distribute oil.
  5. Wipe away any excess oil to prevent staining fabric.

Timing Adjustment

Correct hook-to-needle timing is critical for stitch formation.

Symptoms of Bad Timing: Skipped stitches, thread breakage, poor loop formation.

1. Remove needle plate, bobbin case, and hook.
2. Turn handwheel to bring needle bar to its lowest position.
3. The needle eye should be approximately 1/16" (1.5mm) above the tip of the hook when the hook point is at its closest to the needle.
4. To adjust: Loosen the setscrew on the handwheel. Slightly turn the handwheel on the shaft to advance or retard the hook timing. Retighten setscrew firmly.
5. Reassemble and test with scrap fabric.

Stitch Length & Feed Dog Adjustment

The stitch length regulator is connected to the feed dog mechanism.

1. Stitch Length Dial: Turning the dial changes the distance the feed dogs move the fabric.
2. Feed Dog Height: The feed dogs should protrude just above the needle plate surface (approx. 0.8mm) when in the raised position. Adjust via screw under the needle plate.
3. Feed Dog Timing: The feed dogs should start moving the fabric just after the needle rises out of the material. This is factory-set and rarely needs adjustment.

Tip: If stitches are uneven, check for lint or thread jammed under the feed dogs.

Needle & Bobbin Case Setup

Proper installation is essential for smooth operation.

1. Inserting Needle: Loosen needle clamp screw. Insert a 15x1 needle with the flat side facing the rear. Tighten screw firmly.
2. Winding Bobbin: Place thread spool on pin, guide thread through tension spring, and wind onto Class 66 bobbin evenly.
3. Inserting Bobbin: Place bobbin in case with thread unwinding clockwise. Pull thread into tension spring slot. Insert case into hook until it clicks.
4. Drawing Up Bobbin Thread: Hold needle thread, turn handwheel toward you one full rotation to catch bobbin thread and pull it up through the needle plate.

WARNING! Ensure needle is not bent. A damaged bobbin case can cause tension problems.

Upper & Lower Tension Adjustment

Balanced tension creates a lockstitch with the knot hidden between fabric layers.

Upper Tension: Turn the numbered dial clockwise to increase tension (higher number), counter-clockwise to decrease. Standard setting is around 4-5.

Lower (Bobbin) Tension: Adjust the small screw on the side of the bobbin case. Turn slightly clockwise to increase tension. CAUTION: Small adjustments make a big difference.

Test: Sew on two layers of medium-weight fabric. If top thread loops on bottom, increase top tension or decrease bobbin tension. If bobbin thread loops on top, do the opposite.

Motor & Belt Maintenance

The motor is mounted internally at the base of the machine.

• Belt Inspection: Check for cracks, glazing, or looseness. A worn belt will slip and cause inconsistent speed.
• Belt Replacement: Loosen motor mounting screw to create slack. Remove old belt and install new one. Adjust motor position until belt has about 1/2 inch of play when pressed midway between pulleys. Tighten motor screw.
• Motor Brushes: Access brushes via caps on the motor. If worn shorter than 1/4 inch, replace with Singer part #XXXXX. Ensure brushes move freely in their holders.
• Motor Lubrication: Most motors are permanently lubricated. Do not oil the motor unless specified.

Troubleshooting Common Issues

Symptom	Possible Cause	Corrective Action
Machine won't run	Plugged out, foot control issue, jam	Check power, inspect foot control cord, remove jammed thread, check for seized parts.
Skipped stitches	Wrong/bent needle, bad timing, burred hook	Replace needle, check/adjust timing, polish or replace hook.
Thread breaking	Incorrect threading, tension too high, rough spots	Rethread completely, reduce tension, check for nicks in thread path.
Uneven feed	Lint under feed dogs, worn feed dogs, incorrect pressure	Clean thoroughly, replace feed dogs, check presser foot pressure spring.
Noisy operation	Lack of oil, lint, worn gears/bearings	Clean and oil thoroughly. If noise persists, inspect gears for wear.

Note: Persistent mechanical issues may require professional service.

Parts List & Diagrams

Refer to official Singer parts diagrams for the 348 model. Common replacement parts include:

• Needles (15x1)
• Class 66 Bobbins (metal)
• Bobbin Case (Part #XXXXX)
• Drive Belt
• Motor Brushes
• Presser Foot
• Slide Plate
• Oscillating Hook Assembly

For genuine Singer parts and detailed exploded-view diagrams, contact an authorized Singer service center or visit the Singer website.

Service Support: For technical assistance, contact Singer Customer Service or a certified sewing machine technician.